Hot Cracking Susceptibility Research Articles

Alternatives to Reduce Hot Cracking Susceptibility of IN718 Casting Alloy Laser Beam Welds with a Mushroom Shape

Reducing hot cracking is essential for ensuring seamless production of nickel superalloys, which are extensively used in welded structures for aircraft engines. The prevalence of hot cracking in precipitation-strengthened alloy 718 is primarily governed by two factors: firstly, the chemical composition and the coarse microstructure formed during solidification, and secondly, the activation of hot cracking mechanisms, which is particularly critical in mushroom-shaped welding morphologies. In this study, different nickel-based superalloys welded using laser beam welding (LBW), more specifically bead on plate welding (BoP), specimens are compared. The cracking susceptibility of both wrought and two investment casting 718 alloys with tailored chemical compositions is examined through the application of both continuous and pulsed LBW. Additionally, various pre-weld treatments, including with and without Pre-HIP (hot isostatic pressing), are analyzed. The influences of chemical composition, LBW parameters and pre- and post-welding treatments on both internal and external cracks determined by conventional and advanced non-destructive tests are studied. A clear reduction of hot cracking susceptibility and overall welding quality improvement was observed in a tailored 718 alloy with relatively high Ni (55.6% wt) and Co (1.11% wt) contents.

Microstructure evolution and crack prevention in TiC nanoparticles-enhanced 2195 Al[sbnd]Li alloy joint fabricated by coaxial fiber-diode hybrid laser wire-feeding welding

High hot crack susceptibility (HCS) in laser-welded 2195 AlLi alloy poses a considerable challenge that restricts its widespread application. Nonetheless, the utilization of advanced laser beam shaping technology and nano-technology presents novel approaches to modify the melting and solidification procedures, enabling precise control over microstructure and crack. Herein, a crack-free and well-formed weld seam (WS) of 2195 AlLi alloy was successfully achieved by employing a coaxial “Gussies + Flat top” hybrid laser source and introducing TiC nanoparticles. The contrast experiments were performed to comprehensively investigate the effect of TiC nanoparticles on the microstructure evolution and hot cracks in welded joints of 2195 AlLi alloy. The results indicated that the crack propagation occurs linearly along the grain boundaries of columnar dendrites and laterally along the grain boundaries of equiaxed dendrite utilizing the conventional AlCu filling wire. With the assistance of TiC nanoparticles, the crack-free WS with homogeneously equiaxed dendrites is facilitated. The elimination of cracks by introducing TiC nanoparticles is attributed to the ameliorative liquid feeding channels, grain refinement, as well as the the pinning effect of TiC nanoparticles.

Unveiling the origin of severe hot cracking susceptibility in Al-Li alloys

This work reports a new mechanism for the severe hot cracking susceptibility (HCS) of cast Al-Li alloys. In contrast to the narrow vulnerable solidification temperature range increasing cracking resistance, alloys with high Li content or solute content, which are supposed to show lower HCS, instead exhibit anomalously high HCS. Results reveal that this phenomenon arises from a variety of Li-rich inclusions distributed in the interdendritic paths, including Li2O, LiAlO2, Li2CO3 and LiH. These particles can form spontaneously with O2, CO2 and H2O during solidification, deteriorating the interfacial bonding capacity. Meanwhile, the occupation of refilling channels for residual liquid increases the potential risk of microcrack initiation. These unfavorable factors promote the rapid propagation of cracks from the casting surface inward, which eventually leads to catastrophic fractures.

Crack mitigation strategies for a high-strength Al alloy Al92Ti2Fe2Co2Ni2 fabricated by additive manufacturing

Laser powder bed fusion is capable of fabricating aluminum (Al) alloy parts with great geometrical flexibility and rapid prototyping for various industries. However, high strength Al alloys generally suffer from solidification cracks due to the steep thermal gradient associated with the laser fusion process. Here, we report several strategies to mitigate the hot crack susceptibility of a high strength Al92Ti2Fe2Co2Ni2 alloy. Routine processing parameter optimization based on varying laser power and scanning speed has to trade off porosity for producing crack-free parts, making it not suitable for load-bearing structural applications. Furthermore, secondary printing parameters, including laser strip length, laser defocus, scanning strategies, etc., improved printability but were insufficient to eliminate all the cracks. Crack morphology and residual stress measurements indicate that the cracks are generated in the solid state driven by large tensile residual stress, instead of solidification cracking or liquation cracking. Thus, an attempt was made to alleviate the residual stress in a controlled manner. By properly introducing a compliant, sacrificial, scaffold support structure to regulate crack propagation, near fully dense, crack-free parts could be successfully printed. The results were further verified by micro computed tomography, showing that cracking could be arrested in the support before propagating through the parts. This method can be readily applied to other alloy systems without modifying the chemistry.

Triplex steel powder design to avoid hot cracking in laser-powder bed fusion using computational thermodynamics

High manganese steels offer exceptional combinations of high strength and ductility, resulting in weight reduction when utilized in structural applications. Nevertheless, the conventional manufacturing routes of these steels is hindered by many production problems. Additive Manufacturing (AM) has emerged as a reliable solution to fabricate thin or complex shape compounds using these steel grades. Indeed, several studies have demonstrated the success to fabricate high manganese twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) steels by laser-powder bed fusion (L-PBF). However, a recent study on a high manganese triplex steel composition has revealed the occurrence of both hot cracking and micro segregation during rapid solidification in L-PBF, highlighting potential processability issues of this family of high alloy steels. In this study, the hot cracking susceptibility of different triplex steels is evaluated, focusing on the impact of the composition on the solidification paths and final microstructures. A Computer Coupling of Phase Diagrams and Thermochemistry (CALPHAD) approach is employed to predict alloy-dependent hot cracking susceptibility so as to establish guidelines for preventing hot cracking and provide insights into alloy design for AM. Microstructural observations are used to determine the accuracy of CALPHAD predictions in terms of elemental segregation, phase formation and hot cracking susceptibility. To this end, scanning transmission electron microscopy is used to evaluate elemental segregation at grain boundaries, while the local distribution of phases and their relative amount is measured by electron backscattered and X-ray diffraction, respectively. Hot cracking susceptibility is experimentally evaluated by measuring the length of cracks (when present) in the cross section of printed specimens.

Multiphysical simulation of hot cracking in Laser-Based Powder Bed Fusion

This study extends an existing comprehensive computational framework to gain insight on hot cracking in the simulation of laser-based additive manufacturing via powder bed fusion. A novel approach to predict hot crack susceptibility based on vapor cavitation, building on a conceptual model akin to the Rappaz-Drezet-Gremaud criterion is introduced. Unlike conventional practices involving ex-situ evaluation of a criterion, the proposed model emerges implicitly from the underlying multiphysical modeling framework. The model exhibits sensitivity to variations in both material attributes (e.g., alloy composition) and processing conditions (e.g., laser beam shape or scanning strategy). Furthermore, non-equilibrium solidification is incorporated in the underlying Mass-of-Fluid framework, and a model for multi-layer printing is introduced to extend the length and time scale, enabling the derivation of detailed thermal histories on near-part-scale level. Consequently, the framework proves pivotal in optimizing process parameters, transcending the limitations inherent in conventional single melt track computational experiments.

Comprehensive Analysis of Microstructure and Hot Deformation Behavior of Al-Cu-Y-Mg-Cr-Zr-Ti-Fe-Si Alloy

Low sensitivity to hot cracking is very important not only for casting but also for ingots of wrought alloys. Doping of Al-Cu-(Mg) alloys by eutectic forming elements provides an increasing resistance to hot cracking susceptibility, but it also leads to a decrease in plasticity. The quasi-binary alloys based on an Al-Cu-REM system with an atomic ratio of Cu/REM = 4 have a high solidus temperature, narrow solidification range and fine microstructure. The detailed investigation of microstructure, precipitation and hot deformation behavior, and mechanical properties of novel Al-Cu-Y-Mg-Cr-Zr-Ti-Fe-Si alloy was performed in this study. The fine Al8Cu4Y, needle-shaped Al11Cu2Y2Si2, compact primary (Al,Ti)84Cu6.4Y4.3Cr5.3 and Q (Al8Cu2Mg8Si6) phases were identified in the as-cast microstructure. Near-spherical coarse Al3(Zr,Y) and fine Al45Cr7 precipitates with a size of 60 nm and 10 nm were formed after 3 h of solution treatment at 580 °C. S′(Al2CuMg) precipitates with an average diameter of 140 nm, thickness of 6 nm and calculated volume fraction of 0.033 strengthened 36 HV during aging at 210 °C for 3 h. Three-dimensional hot processing maps demonstrated an excellent and stable deformation behavior at 440–540 °C and strain rates of 0.01–10 s−1. The rolled sheets had a good combination of yield strength (313 MPa) and plasticity (10.8%) in the recrystallized at 580 °C, with water quenched and aged at 210 °C for a 3 h state. The main calculated effect in the yield strength was contributed by Al45Cr7 precipitates.

Hot Crack Susceptibility Studies on High-Nitrogen Stainless Steel

Hot Crack Susceptibility Studies on High-Nitrogen Stainless Steel

Optimization of Ultrasonic-Assisted TIG (UA-TIG) Welding Process Parameters for AA7075 Alloy Joints Using RSM-GA Approach

Abstract The current study investigates the optimized process parameters for low hot cracking susceptibility and the enhanced microhardness of ultrasonic-assisted tungsten inert gas (UA-TIG) welded AA7075 joints. The optimization trials were made using response surface methodology (RSM) and RSM coupled with genetic algorithm (RSM-GA) approaches. Welding process parameters, such as welding current, gas flow rate, presence and absence of ultrasonic vibration (UT), and filler metal were varied to study the hot crack sensitivity and microhardness of the AA7075 welded joints. RSM was used to develop the empirical relationships with a higher coefficient of determination (R2 = .9892 for hot cracking sensitivity and R2 = .9919 for microhardness). Welding current (120 A), gas flow rate (13 L/min) with UT and filler material with hot cracking sensitivity (0 %), and microhardness (117.76 HV) are the optimized process parameters. The experimental results and predicted RSM model at optimal conditions were compared to the predicted RSM-GA model for minimal hot cracking sensitivity and maximum microhardness. A good agreement between the experimental results and the predicted RSM model confirms the soundness of the developed RSM-GA model.

Effect of chemical composition on hot cracking susceptibility (HCS) using various hot cracking criteria

The paper aims to evaluate the effect of chemical composition on the Hot Cracking Susceptibility (HCS) using mechanical and non-mechanical hot cracking criteria during solidification. The criteria were SKK as a mechanical criterion. Feurer, Clyne Davis, and Katgerman as non-mechanical criteria. The criteria were implemented at various parameters to evaluate their abilities in the hot cracking susceptibility (HCS) prediction at varied chemical composition. In this study, The Mg content was varied in Al9Zn (1, 1.5, 2, 2.5 %wt.) Mg2Cu alloys and Cu content in Al9Zn2Mg (1, 1.5, 2, 2.5 %wt.) Cu alloys. The validation of the result is also conducted by comparing with the experimental data. Based on Feurer criterion, The hot cracking initiates at lower temperature and at higher critical rate of feeding and shrinkage with Cu content, and the hot cracking initiates at higher temperature with Mg content, and it initiates at higher critical rate of feeding and shrinkage from 1 up to 1.5 of Mg, and the critical rate of feeding and shrinkage remains constant from 1.5 up to 2.5 of Mg. Based on Clyne & Davies, the HCS decreases with Cu content from 1 up to 2 of Cu, and it increases from 2 up to 2.5 of Cu. The HCS decreases with Mg content from 1 up to 2 of Mg, and it remains constant from 2 up to 2.5 of Mg. Based on Katgerman criterion, the HCS decreases with Cu content from 1 up to 1.5 of Cu, it increases from 1.5 up to 2 of Cu, and it decreases from 2 up to 2.5 of Cu. The HCS decreases sequentially with Mg content. Based on SKK criterion, the HCS curves shift to the right with Cu content which means that the hot cracking initiates at lower temperature, and the HCS curves shift to the left with Mg content which means that the hot cracking initiates at higher temperature with Mg content. The Feurer, Clyne & Davies, and some specific range for SKK criteria are in agreement for the effect of Cu content on HCS of alloys, and Katgerman and some specific range for Clyne&Davies criteria are in agreement for the effect of Mg content on HCS of alloys.

Hot Cracking Behaviors of Mg-Zn-Er Alloys with Different Er Contents.

The hot cracking behaviors of Mg-5Zn-xEr (x = 0.83, 1.25, 2.5, 5 wt.%) alloys are investigated by optimized hot cracking experimental apparatus, optical microscope, and scanning electron microscope, such as contraction behaviors, feeding behaviors, and permeability characteristics. It is found that the solid phase fraction at hot crack initiation and within the freezing range both increased with increasing Er contents up to 2.5 wt.% and then decreased at 5 wt.% Er content. The Mg-5Zn-5Er alloy exhibits the lowest solid phase fraction (87.4%) and a reduced freezing range (74.2 °C), which leads to more effective liquid feeding in the latter stages of solidification. Combined with the grain size, the permeability of the mushy zone, and fracture morphology, the overall permeability is optimal in the Mg-5Zn-5Er alloy, which is beneficial for feeding the cavities and micro-pores. Meanwhile, a large amount of W phase precipitated by the eutectic reaction (L→α-Mg + W phase), which facilitates healing of the incurred cracking. Conversely, the Mg-5Zn-2.5Er alloy shows inferior feeding ability due to the lowest solid phase fraction (98.3%), wide freezing range (199.5 °C), and lowest permeability. Therefore, the Mg-5Zn-2.5Er alloy exhibits maximal hot cracking susceptibility, and the Mg-5Zn-5Er alloy exhibits minimal hot cracking susceptibility. This work provides guidance for improving the hot cracking resistance of cast Mg-Zn-Er alloy and enables an understanding of the hot cracking behaviors of Mg-Zn-RE alloys.

A new Al-Cu alloy for LPBF developed via ultrasonic atomization

Wrought 2xxx aluminum alloys are difficult to process by laser powder bed fusion (LPBF) because of the hot cracking susceptibility caused by their large solidification range. Although several studies on Ti and Zr additions to 2xxx Al-Cu alloys show improved processability in LPBF, only few explore the addition of alternative alloying elements such as Cr and Fe. There is thus little knowledge on the ability of these elements to avoid hot cracking. In the present work, a new Al-Cu alloy with Ti, Cr and Fe additions is put forward and the mechanisms impeding hot cracking formation are analyzed. (Al, Cr)3Ti_L12 precipitates are formed during the solidification process, promoting heterogenous nucleation and grain refinement. Cr not only contributes to solid solution strengthening but also supports the stabilization of the Al3Ti metastable cubic phase. The addition of near-eutectic Fe decreases the solidification range, further reducing the susceptibility for hot cracking. Nano-hardness mapping reveals the solidification path of the alloy, with higher values associated with the highly dense areas of precipitates forming at the melt pool boundaries. A novel printable alloy with hardness values exceeding those of existing Al alloys for LPBF was designed.

Microstructure and mechanical properties of pseudo binary eutectic Al–Mg2Si alloy processed by laser powder bed fusion

The traditional wrought Al–Mg–Si alloys fabricated via laser powder bed fusion (LPBF) are prone to hot cracks, unless adding grain refiners in as-LPBFed Al alloys. In this work, the Al-9.6 wt.%Mg-4.9 wt.%Si (equivalent to pseudo binary eutectic Al-13.3 wt.%Mg2Si) alloy with low solidification range and hot-cracking susceptibility was successfully processed by LPBF. The as-LPBFed alloys have reached a high relative density of 99.3% at the VED of 129.6 J/mm3. The microstructures were featured by fine α-Al grains and cellular eutectic Mg2Si, accompanied by a high number density of dislocations, coherent GP zone and α-Al12(Fe,Mn)3Si phases. The as-LPBFed Al-13.3Mg2Si alloy exhibited the high ultimate tensile strength of 557 MPa, yield strength of 439 MPa and elongation of 2.9%. In addition to the grain refinement and dislocation strengthening, the strength enhancement is mainly ascribed to the dispersion strengthening from the divorced nanosized eutectic Mg2Si. The results demonstrate that manipulation of alloys at near eutectic composition is effective to achieve high strength Al–Mg–Si alloys processed by LPBF.

Laser powder bed fusion of Zr-modified Al-Cu-Mg alloy: Processability and elevated-temperature mechanical properties

• Hot-cracking was affected by the volume fraction and size of the equiaxed grains. • The nucleation potency of L1 2 -Al 3 Zr phase was evaluated based on the E2EM model. • Both as-fabricated and T6-treated samples exhibited excellent YS from 150 °C to 350 °C. Zr modification is an effective method for improving hot-cracking resistance and elevated-temperature mechanical properties during laser powder bed fusion (L-PBF) of traditional medium and high strength wrought aluminum alloys. This study investigated the L-PBF processability and elevated-temperature mechanical properties of a Zr-modified 2024Al alloy. It was found that the hot-cracking susceptibility increased with the increased scanning speed, which was in reasonable agreement with the modified Rappaz–Drezet–Gremaud criterion. Furthermore, the primary L1 2 -Al 3 Zr precipitates, which acted as efficient nucleation sites, precipitated at the fusion boundary of the melt pool, leading to the formation of a heterogeneous grain structure. The yield strength (YS) of the as-fabricated samples at 150, 250, and 350 °C was 363, 210, and 48 MPa, respectively. Despite the slight decrease to 360 MPa of the YS when tested at 150 °C, owing to the additional precipitate strengthening from the L1 2 -Al 3 Zr precipitates, the YS achieved yield strengths of 253 and 69 MPa, an increase of 20.5% and 30.4%, when tested at 250 and 350 °C, respectively. The yield strengths in both the as-fabricated and T6-treated conditions tested at 150 and 250 °C were comparable to those of casting Al-Cu-Mg-Ag alloys and superior to those of traditionally heat-resistant 2219-T6 and 2618-T6 of Al-Cu alloys.

Assessment of the Hot-Cracking Susceptibility of Welded Joints of the 7CrMoVTiB10-10 Bainitic Steel Used in Heat Exchangers

Bainitic steel containing approx. 2.25% Cr and 0.6 Mo with micro-additions of V, Ti, and B, designated as 7CrMoVTiB10-10 (T/P24), is one of the new construction materials used in new supercritical power units. The weldability of 7CrMoVTiB10-10 is defined as the hot-cracking susceptibility, it should be stated that the hot cracking in the welded joints of 7CrMoVTi10-10 is determined by phenomena occurring in the high-temperature brittleness range (HTBR). In this work, the HTBR is established for both the base material and welding conditions, taking into account the critical temperature-strain intensity (CST) and the critical strain speed (CSS). The HTBR for 7CrMoVTi10-10 is 122 °C wide and covers temperatures from 1394 °C to 1516 °C. Under imposed deformation conditions (typical during welding), the HTBR extends to a width of 293 °C towards lower temperatures, i.e., from 1516 °C to 1223 °C. The CSS = 0.83 1/s, the CST = 0.003 1/°C, and the Rf index = 0.12, which can be adopted as the criteria for the susceptibility of 7CrMoVTiB 10-10 to hot cracking under imposed deformation conditions. The hot cracking in 7CrMoVTiB10-10 occurs as a result of the loss of cohesion by the thin liquid layer crystallising between the growing weld crystals. Such cracks appear when the CSS or the CST is exceeded within the HTBR.

Composition design of low hot-cracking susceptibility of Al-Zn-Mg-Sc alloy and its formability during laser additive manufacturing

Aluminum‑zinc‑magnesium (Al-Zn-Mg) alloy that is fabricated by using traditional plastic processing is not suitable for laser additive manufacturing due to its tendency to crack. To design aluminum alloy with low hot cracking tendency and is suitable for laser additive manufacturing, an aluminum‑zinc‑magnesium‑scandium (Al-Zn-Mg-Sc) system is proposed in this study. A theoretical model is used to predict the hot cracking susceptibility of this system. The optimized alloy composition is Al-6.5Zn-2.2 Mg-0.4Sc and its formability during laser melting deposition (LMD) is then investigated. An anisotropic microstructure is observed in the alloy: columnar grains with a [001]//Z texture in the building plane and equiaxed grains in the scanning plane. Pores are the main metallurgical processing defects, but a few hot cracks can be seen in the laser melted (Al-Zn-Mg-Sc) alloy. The mechanical properties can be significantly improved through heat treatment: the ultimate tensile strength of the alloy is increased from 258.7 MPa to 390.3 MPa, and elongation of the alloy is increased from 13.1% to 29.6% after solution and aging treatments, thus indicating that the alloy has good plasticity.

Surface modification of high-Mn steel via laser-DED: Microstructural characterization and hot crack susceptibility of clad layer

High-Mn steels have been intensively developed in steel industries due to their excellent mechanical properties and low density. However, the weak surface properties of these steels significantly limited the production and application scope. Various surface modification solutions were proposed, but they were not suitable for mass production or degraded the physical properties of base materials. Here, a new solution is proposed for mass-producing an ultra-thin multilayer clad steel sheet using an additive manufacturing process without changing the superior mechanical property of the high-Mn steel. Functionally graded multilayers from high-Mn steel to high-strength low-alloy steel have been deposited onto a high-Mn steel slab by a powder-fed type additive manufacturing process to improve its surface properties. During the deposition at high laser intensities, solidification cracks appeared in the first and second layers. The hot crack susceptibility of the functionally graded multilayers was elucidated by employing thermodynamic solidification calculation and thermo-mechanical analysis.

Evaluation of narrowed weld pool shapes and their effect on resulting potential defects during deep penetration laser beam welding

This study presents mechanisms of the evolution of a narrowed region in the weld pool center during deep penetration laser beam welding. In numerous numerical studies presented in this study, it was also found that the local reduction of the weld pool size can cause detrimental effects on the melt flow behavior and the resulting properties of the welds. A particularly large influence of this effect was identified in three aspects. First, the local variation of the solidification sequence of the weld pool causes an increase in the hot-cracking susceptibility due to a locally delayed solidification. Second, it was proven that a change in the local length and width of the weld pool is associated with an adverse impact on the potential flow routes of the molten material that induces stronger local variations of its solidification. Thus, the element mixing, e.g., during the welding with filler materials, is blocked. This leads to a nonhomogeneous chemical composition of the final weld and can cause undesired effects on the final material properties. Finally, another observed effect is related to the reduced ability of process pores to reach the top surface. As this type of porosity is usually produced around the keyhole tip, the change of the fluid flow regime above this area plays a significant role in determining the final path of the pores until the premature solidification in the middle of the weld pool captures them. This study summarizes mainly numerical results that were supported by selected experimental validation results.

Effects of Metal and Fluoride Powders Deposition on Hot-Cracking Susceptibility of 316L Stainless Steel in TIG Welding

This study aims to investigate the effects on the hot cracking susceptibility of fluoride powders such as CaF2, NaF, LiF, and metal powders such as Mn, Ti, Nb and mixed Ti-Nb deposited on the 316L stainless steel during the TIG (Tungsten Inert Gas) welding process. A self-restraint hot cracking bench test using specimens of trapezoidal shape and 3 mm of thickness was selected. The obtained results of the weldability with the different powders were compared with those obtained with the conventional TIG parent-metal weld. The susceptibility to hot cracking was evaluated by the length of the crack and by the critical width at the end of the crack propagation. The formed cracks were first revealed by the liquid penetrant test, and then the surfaces of cracks were observed and analyzed by SEM-EDS-XRD tools. Among the powders tested, single Nb powder and the mixed flux of 80% Nb + 20% Ti exhibited the lowest crack length. The crack propagation ended at 22 mm of length and 30.8 mm of width. The analyses of the fracture surfaces of cracks revealed the presence of Niobium carbide (Nb2C), titanium, chromium, niobium oxide (TiO0.6Cr0.2Nb0.0202) complex compounds and cementite (Fe3C) at the interdendritic zones.

Correction to: An Updated Index Including Toughness for Hot-Cracking Susceptibility

Correction to: An Updated Index Including Toughness for Hot-Cracking Susceptibility